Boiling water nuclear reactors typically include a reactor core located within a reactor pressure vessel (RPV). A known RPV includes a substantially cylindrical shell. The shell, for example, can be about twenty feet in diameter and about seven inches thick. The cylindrical shell is closed at its top end by a removable top head. The top head is removable so that components, such a fuel bundles, located in the RPV can be accessed. The RPV cylindrical shell is closed at its bottom end by a dome shaped bottom head assembly welded to the shell. A plurality of openings, sometimes referred to as penetrations, are formed in the bottom head dome so that components can extend within the RPV.
For example, in-core guide tubes (ICGT) and in-core housings (ICH) are long tubes which are welded together end to end to form an assembly, and the ICGT and ICH support and protect the in-core instrumentation. The bottom end of the ICH penetrates the bottom of the reactor pressure vessel and is welded to the bottom head dome. Welding the ICH to the bottom head dome provides rotational, vertical and horizontal support of the assembly. The top end of the ICGT penetrates the core plate, and the ICGT makes a slip fit with the core plate. The slip fit with the core plate provides only horizontal support of the assembly. The distance between upper and lower end supports is approximately 16 feet.
During operation, fluid flows along the outside of the ICH/ICGT assembly. Particularly, fluid flows parallel to the axis of the assembly as well as across the assembly. Such flow causes flow induced vibrations (FIV) in the assembly. If unsupported between the ends, the ICH/ICGT assembly would experience vibration levels above acceptable limits.
Known restraints for the ICH/ICGT assembly include a lattice arrangement of bolted plates which tie the ICGTs together. In at least one known reactor, two sets of lattices are attached to the shroud and tie the ICGTs together to reduce the FIV to an acceptable level. Tying the ICGTs together changes the natural frequency of the ICH/ICGT assemblies. Thus, the FIV may be reduced by connecting the ICGTs.
The known restraint structures are expensive to fabricate and are difficult to install. It would be desirable to provide a simpler but effective restraint configuration for ICH/ICGT assemblies. Of course, such a restraint should not add significant fabrication and maintenance costs to the reactor.